1. Field of the Invention
The present invention generally relates to a developing device for use in an electrophotographic copying machine, a printer or the like and more particularly, to a developing device which uses a monocomponent developer, and is capable of easily moving a developing means in and out of contact with a photoreceptor.
2. Description of the prior Art
Conventionally, Japanese Patent Laid-open Application No. 52-143831 discloses one such monocomponent developing device, in which upon supply of the non-magnetic toner onto the surface of an elastic deveoping roller, a blade is pressed against the developing roller so that a thin layer of charged toner may be formed on the peripheral surface thereof and a toner image is, then, formed by bringing the thin layer of the charged toner into direct contact with the surface of the photoreceptor.
Japanese patent Laid-open Application No. 55-77764 discloses another monocomponent developing device, in which an electrically conductive thin film is arranged on the surface of a developing roller consisting of an electrically conductive soft elastic foamed member. In this developing device, the toner is caused to adhere electrically to the surface of the developing roller with the use of a magnetic brush; and the toner is caused to adhere to an electrostatic latent image through contact between the developing roller and the surface of the photoreceptor to form the toner image.
However, even in both of these methods, the formation of a thin layer of the charged toner necessitates keeping the blade in contact with the surface of the developing roller under a certain pressure greater than a perdetermined one. Therefore, the developing roller is required to be relatively high in hardness. On the contrary, to prevent the photoreceptor from being damaged or the image from being smeared on a contact portion between the developing roller and the photoreceptor, the developing roller is required to very softly contact the photoreceptor. But a developing roller which may satisfy both of these requisites cannot be obtained since these two requisites are completely opposed. Furthermore, in particular, in the case where a peripheral speed of the developing roller is to be differentiated from that of the photoreceptor, the image formed on the photoreceptor tends to be unclear.
On the other hand, recently, the monocomponent developing devices with the above-mentioned method for developing through contact with the photoreceptor are used for the development of exchanging the color of an image. In this development, when two developing devices simultaneously contact with a photoreceptor, there arises the problem that the developer of one developing device intrudes into the developer of another developing device.
For resolving this problem, Japanese Patent Laid-open application No. 62-15574 and No. 62-15575 conventionally disclose methods, in which only the necessary developing device for development is caused to contact with a photoreceptor following the displacement of another unnecessary developing device.
Meanwhile, even in the case of a single color development using a single developing device, the developing device must be taken out in a direction of the axis of a photoreceptor for the purpose of exchanging the developing device to change the color of an image and the maintenance of an apparatus. For this reason, a developing device is required to be moved away from the photoreceptor in order to release the contact between a developing roller and a photoreceptor.
However, in order to release the contact between a photoreceptor and a developing roller in conventional developing devices, the whole developing device was required to be moved away from the photoreceptor. There arises the problem that the construction of an apparatus of this type become quite complicated.